Aug 8 (Reuters) - The main union at Chile’s Caserones copper mine said on Wednesday that a last round of labor negotiations with mine operator Lumina Copper had broken down and that a strike was imminent.

The 300-member worker’s union said Lumina, which is controlled by a partnership of JX Holding and Mitsui Mining , had taken an “intransigent” position to their demands during a final period of government-mediated talks.

“Negotiations with the company under mediation have failed. Management wanted ... to put an end to the negotiations by offering us a bonus, but they weren’t listening to our demands,” union president Nelson Iribarren told Reuters.

If Lumina doesn’t improve its contract offer by Friday, a strike would begin next Tuesday, Aug. 14, Iribarren said.

The company was not immediately available to comment.

A looming strike at Caserones comes as workers at Chile’s Escondida copper mine, the world’s largest, enter a final phase of negotiations with miner BHP, heightening uncertainty in copper markets amid growing trade tensions between the U.S. and China.

The union at Caserones had voted to reject Lumina’s final contract offer in late July.

The mine produced 122,800 tonnes of copper in 2017, just over two per cent of Chile’s total annual copper output. (Reporting by Antonio de la Jara; writing by Dave Sherwood; editing by Richard Pullin)